DE
DANIELS
ELECTRONICS
2-4
Enhanced AM/FM Synthesizer Instruction Manual OS(R/T)-3(A/H) 29 - 470 MHz
diodes D1 and D2. Coarse frequency adjustment is provided by varactor diodes D6, D7. Select
capacitor values C40 or C23 are chosen to position the operating frequency in one of two bands;
118 - 138 MHz, and 139.4 - 159.4.MHz. Varactor diodes D1 and D2 provide fine oscillator
frequency control. PLL voltage feedback control from the output of the lowpass loop filter controls
the VCO frequency through the reverse biasing of diodes D1 and D2. The PLL control voltage can
range from
≈
+1.0 Vdc to +4.7 Vdc and is normally set to
≈
+3.3 Vdc for the whole range of
synthesizer frequencies. Setting the PLL control voltage set point (TP4) is made by adjusting the
capacitance of varactor diodes D6, D7, which are controlled by a microcontroller U18 through a
digital to analog converter U17. The microcontroller U18 controls the test point TP4, the output of
the PLL Low Pass Filter, and adjusts the output voltage of D/A converter so that the TP4 voltage is
approximately is 3.3Vdc at any frequencies. The microcontroller than goes sleep in order to keep
the power consumption at the low level.
The PLL lowpass filter is formed from select component values C37, C38, C39, C45, R32, and
R36. The loop filter response is optimized for switching time, noise, and modulation requirements
specific to each sub-band within the 118 - 159.4 MHz frequency range. The select component
values (including the loop filter) can be found in tabular format as part of the VHF OS(R/T)-
3(A/H) 118 - 159.4 MHz Analog Board Schematic diagram.
RF output power is taken from the source of Q5 and amplified/buffered by Q6. U11 provides
further amplification and isolation while delivering
≈
+10 dBm to a five pole lowpass output filter
formed from C53, C58, C59, L11, and L12. The five pole lowpass output filter provides a 190
MHz low pass cutoff frequency which reduces harmonic output to acceptable levels. SMB
connector J2 provides interconnection to the companion transmitter or receiver with an output level
of
≈
+5 dBm.
2 . 2 . 4
128 - 174 MHz Analog Board Circuitry
Refer to the "OS(R/T)-3(A/H) 128 - 174 MHz Analog Board Schematic Diagram" located in
section 4 page 11 of this manual.
Field effect transistor Q5 provides a VHF negative resistance amplifier/oscillator that is tuned on
frequency by the combination of tapped resonator L5 and a total capacitive reactance presented
across L5 through capacitors C40 and/or C23 (Select), variable capacitor C24, and varactor diodes
D1 and D2. Fine frequency adjustment is provided by multi-turn trimmer capacitor C24. Select
capacitor values C40 or C23 are chosen to position the operating frequency in one of two bands;
128 – 152.6 MHz and 150 - 174 MHz. Varactor diodes D1 and D2 provide oscillator frequency
control. PLL voltage feedback control from the output of the lowpass loop filter controls the VCO
frequency through the reverse biasing of diodes D1 and D2. The PLL control voltage can range
from
≈
+1.0 Vdc to +7.0 Vdc and is normally set to
≈
+4.5 Vdc for the nominal programmed
synthesizer center frequency. Setting the PLL control voltage set point (TP4) is made by adjusting
fine frequency variable capacitor C24. External baseband frequency modulation is provided
through connection P1 and a voltage divider network formed by R21 and R22. A large signal
